Electrochemical batteries have for some time been used as a means to power a variety of electronic consumer products. Conventional batteries are usually of the type having an anode, a cathode, a porous separator to maintain physical separation between the anode and cathode, and a suitable electrolyte supplying a source of positive and negative ions which migrate between the anode and cathode during use.
When used correctly (assuming no inherent defect in the battery), there is little risk that the battery will present a safety hazard to its user. However, when used incorrectly (as by a forced short-circuit condition caused by reversing the battery's polarity during use) and/or when a defect in the battery is present (as by a short-circuit condition due to the anode and cathode coming into physical contact with one another), there is a risk that uncontrolled chemical reaction of potentially explosive magnitude may occur within the battery. This risk is particularly acute for batteries employing a highly electropositive anode, for example, lithium, although the risk may still be present for more traditional electrochemical batteries, for example, nickel-cadmium cells. While a battery manufacturer can implement quality control procedures to minimize defectively manufactured batteries and prevent them from reaching consumers, there is little that can be done to ensure absolutely that batteries will be used correctly by their ultimate users.
Various proposals already exist to minimize uncontrolled thermal reactions in electrochemical battery cells as evidenced by U.S. Pat. No. 4,650,730 to Lundquist et al issued on Mar. 17, 1987; U.S. Pat. No. 4,731,304 to Lundquist et al issued on Mar. 15, 1988; U.S. Pat. No. 4,075,400 to Fritts issued on Feb. 21, 1978; U.S. Pat. No. 4,351,888 to Dampier et al issued on Sept. 28, 1982; U.S. Pat. No. 4,407,910 to Catanzarite issued on Oct. 4, 1983; and U.S. Pat. No. 4,741,979 issued to Faust et al on May 3, 1988.
The Lundquist et al U.S. Pat. Nos. 4,650,730 and 4,731,304, disclose sheet products said to be useful as battery separators, having at least two microporous plies which are coextensively bonded together into a unitary product. When the sheet is subjected to elevated temperatures, as where shorting occurs in an electrical storage battery, one of the plies is intended to melt and transform into a non-porous membrane. This pore closure is intended to shut down the electrical current flow in the battery.
According to Fritts '400, a woven mat assembly contains a plurality of thermoplastic globules which encapsulate a reaction-deactivating "poison". When the internal temperature of the battery reaches a predetermined maximum, the "poison" is released thereby deactivating the chemical reaction.
In Dampier et al '888, the current flow within a battery cell during abnormal operating conditions is limited due to dissolution of an additive material (for example, polyvinyl chloride) in the electrolytic solution. During normal operating conditions, however, this additive material is dispersed throughout the electrolytic solution without adversely affecting current flow within the cell.
The electrochemical cell according to Catanzarite '910 includes an inorganic solid anode-neutralizing agent which, at or near the melting point of the anode, enters into an endothermic, or at most mildly exothermic, reaction with the anode thereby neutralizing the same. At other temperatures, however, the anode-neutralizing agent is non-reactive with all cell components, including the anode.
A battery separator is disclosed in Faust et al '979 as including a porous film (e.g., a microporous film) bearing a porous layer of wax-coated fibers which serves as a thermal fuse. During normal operation, the wax-coated fiber layer does not close the pores of the film.
Notwithstanding these prior proposals in the art, there still exists a need to improve the safety of electrochemical batteries. It is towards satisfying such a need that the present invention is directed.